January 2013: Seven!

Release date: Wednesday, 2nd January 2013

Seven! In our seventh birthday show, we talk to Professor Joanna Haigh about the effect of the Sun on climate change, Stuart rounds up the latest news and we hear what we can see in the January night sky from Ian Morison and John Field.

The News

In the news this month: the youngest young stellar objects, stellar sibling rivalries and planets go missing.

For over fifty years we have understood that the vast stellar seas of gas and dust known as molecular clouds are the birth chambers of stars. The process of producing a star requires that the molecular cloud is large enough to foster cold cores of gas and dust that reach temperatures of just tens of Kelvin. Then, with the help of a pressure front, such as could be produced by a supernova explosion, the material within the cold core can begin to collapse into a small dense envelope of dust containing the first moments of a new star, a protostar, which is encircled by a rotating disk which feeds material from the envelope onto the star's surface. At this point the star is known as a young stellar object (YSO) and will remain in this state for another 100 million years before it throws off its blanket of surrounding dust by igniting its fusion core and ascending out to share its place among its older brethren.

Young stellar objects are broken into four classifications, 0, 1, 2 and 3 where 0 is the youngest and 3 is the oldest. When observing YSOs the majority are noted as being either class 2 or class 3 as this is both where the YSOs are brightest and spend the majority of their lifetimes. It is at some point between class 2 and 3 that nuclear fusion begins. Before nuclear fusion, the protostar is just a highly dense ball of ionised gas heated by gravitational collapse which glows brightly in the infrared part of the spectrum. The earliest stage, class 0, is the poorest understood period of a YSO's life but it is while in this stage that important physics describing the transition from the free gas in a molecular cloud into a fully defined YSO occurs.

Until recently however no telescope has been able to study a class 0 young stellar object for reasons exactly opposite of that mentioned previously; the protostar is both extremely dim since it trapped within a thick veil of dust and it spends less than 10,000 years in this state. However, thanks to the new generation of infrared telescopes, such as SMA and CARMA, a team has managed to measure the accretion disk around a class 0 YSO called L1527. By using Kepler's laws with measurements of the protostars rotating disk the researchers have determined that L1527 is only a fifth of the mass of the cloud of dust which surrounds it. This ratio is directly related to the age of the YSO, as low numbers mean that there has not been much time for dust and gas to fall onto the protostar. Critically, L1527 agrees with expectations made of class 1 YSOs and therefore, as far as can be seen, is well on its way to hosting its own solar system in the future.

Binary star systems account for about half of all the star systems in the Milky Way. For the majority of binary stars they form like their single star cousins, enclosed within an envelope of gas and dust, but with the small difference that binary stars share their primordial cloud with others. However a certain subset of binary stars, known as wide binary stars, offer up a problem to this simple idea because the stellar pair is separated by such a vast distance, of the order hundreds of times the diameter of the solar system, such that there is simply no way they could have formed together in a single cloud. The current model for how wide binaries form assumes that it occurs at a time after the host star cluster has dispersed. At which point the environment becomes calm enough that two distant stars are able to form a tenuous gravitational bond.

A new theory proposed by Reipurth and Mikkola suggests a completely different process which they have determined using computer simulations of star clusters. Their model requires a slight modification to the expectations of a wide binary star system in that it is in fact three stars disguised as two. Initially the assumption is that three stars form in close proximity within a star cluster, all three tightly bound to each other providing them with the necessary security to escape the turbulent conditions they were born into. Then, slowly, over the course of millions of years two of the sibling stars begin to form a closer bond and spiral into an ever tighter orbit around each other while simultaneously pushing the third star further away.

What the model hinges upon though is that all wide binary star systems are in fact three body star systems which at the moment is not supported by observation. However telescopes currently would only be able to distinguish two closely orbiting stars if they are very near to us and this is further complicated because the model predicts that the two close stars may even merge into one. Although this means that the model certainly needs more time to be tested it is worth noting that the nearest neighbours to the Sun, Alpha Centauri A, B and, the closest, Proxima Centauri are all part of single three body wide binary star system.

Finally, researchers using the Haute-Provence Observatory have made follow up observations of some of the brightest confirmed Jupiter-like planets detected by the Kepler Satellite. The team measured the wobble of the stars caused by the orbit of a nearby planet and therefore could make an independent measurement of the confirmed planets using a different technique to Kepler's lightcurve method.

Unfortunately rather than confirm Kepler's findings the follow up measurements actually found 35% of planets were simply false positives caused by other effects. Although Kepler was expected to have some false positive detections the previous estimate before Kepler's launch was a mere 5%. What makes this important is not so much that now the number of confirmed planets will go down but that it indicates that the understanding of what causes the false positives is poor.

The method Kepler uses to detect planets is to observe a star over a period of time and if a planet transits across the disk of the star Kepler will measure a decrease in the stars brightness. What caused the false positives is that there are situations which can cause a stars brightness to decrease even if a planet is not involved. The three main culprits are: A small companion failed star known as a brown dwarf is transiting instead of a planet. Alternatively a much larger star is orbited by a small Sun-like star and brown dwarf binary system which can cause planet like dips in the lightcurve. A final possibility could be that Kepler is observing a very tight binary star system where the plane of orbit is tilted so that the two stars slightly overlap each other as one passes in front of the other. Fortunately it is possible to correct for these problems and already new models are estimating false positive percentages closer to the observed 35%. However it is perhaps slightly concerning that this problem only became apparent after ground based follow up observations because this sort of work is both expensive and limited to only a small number of all the stars observed by Kepler.

The Night Sky

Northern Hemisphere

Orion is prominent in the evening sky. Following the line of his Belt upwards, you reach Taurus. The Hyades Cluster and the nearer star Aldebaran are located here. The Pleiades Cluster is a little further up. Following Orion's Belt downwards brings you to Sirius, the brightest star in the night sky. The open cluster M41 is a few degrees below Sirius. The little-known constellation of Monoceros is to Orion's left, containing little except the Rosette Nebula and the nearest known black hole to Earth (though only one of these two is visible to ordinary telescopes!) Up and to the the left of Monoceros is the star Procyon in the constellation of Canis Minor. Pollux and Castor, the heads of the Gemini Twins, are above Procyon. Close to the legs of the higher Twin is M35, an open cluster. Continuing on, we reach Auriga, which hosts the bright star Capella and the open clusters M36, M37 and M38. Moving down from Pollux, through Castor, brings us to the constellation of Cancer. The Beehive Cluster lies here, and is spectacular through binoculars.

The Planets

Jupiter is high in the east after sunset, reaching its highest point due south around 21:00 UT (Universal Time) at the beginning of the month and around 19:30 UT at the end. It gets to around 60° elevation, which is about as high in the sky as it can ever be. It has a magnitude of -2.7 and is about 5° to the upper-right of the star Aldebaran. Its retrograde (westward) motion continues until the end of the month. Its angular diameter drops from 47 to 43" during January, but its atmospheric bands and larger moons can still be seen using a small telescope.

Saturn rises around 03:00 UT at the beginning of the month and 01:00 at the end. It has a magnitude of +0.6 and an angular diameter increasing from 16.2 to 17" during the month, and lies in Libra. Its rings are now at 19° to the line of sight, making them more visible than they have been for the last six years. However, the planet is not very high in the sky, and so is currently seen through more atmospheric haze. On the 30th, it casts a visible shadow on its ring system as it is at western quadrature (90° from the Sun in the sky).

Mercury can just be spotted to the lower left of Venus before dawn at the beginning of January, and similarly to the lower left of Mars just after sunset at the end, but is otherwise hidden by the Sun.

Mars is still visible just after sunset. It can be seen early in the month at an elevation of 10° in the south-west, about 45 minutes after sunset. By month's end, this decreases to 6°, and the planet's angular size is only 4".

Venus is approaching the Sun in the sky and so is coming to the end of its morning apparition. Its angular size decreases from 10.8 to 10.1" over the month, while its illuminated fraction increases from 95 to 97%, keeping its magnitude at a constant -3.9.

Highlights

This month is the last of the three months in which Jupiter is best seen, and it is high up quite early in the evening. Its North Equatorial Belt is broader than it was a few years ago, and the Great Red Spot is a somewhat pink feature of the South Equatorial Belt.

Saturn and a waning crescent Moon can be seen before dawn on the 30th, below the star Spica in Virgo.

Jupiter is close to the asteroid Vesta in the sky this month (and also between the Hyades and Pleiades Clusters). Vesta is 2° away from the red star Aldebaran and can be seen using binoculars, the best time being around New Moon on the 10th and 11th, when it is very close to a star of similar brightness and so looks like a double star.

Venus appears alongside a thin, waning crescent Moon just before dawn on the 10th.

Mars is close to a thin, waxing crescent Moon just after sunset on the 13th. You may be able to see Earthshine at this time, as sunlight reflected from clouds on the Earth illuminates the dark part of the Moon.

Southern Hemisphere

Jupiter continues to travel through Taurus and is gradually dimming as it moves away from the Earth, while the constellations of Orion and Canis Major are nearby in the northern sky. The head of the V-shape of Taurus is the Hyades Cluster, which appears along with the red star Aldebaran. The Pleiades Cluster, marking the Bull's back, is to the west of the head. Gemini and Cancer are also in the southern sky. The bright stars Castor and Pollux form the heads of the Gemini Twins, and are found in the north-east after sunset. Gemini is at the edge of the Milky Way, and near to Castor are five faint and distant galaxies which can be viewed using a large telescope. The open star cluster M35 is near to the star Eta Geminorum, and can be seen with the naked eye. Binculars or a small telescope reveal the detail of individual stars. Cancer contains five bright stars and the open cluster Praesepe, or the Beehive, at its centre.

Orion dominates the summer sky. Below the three stars of Orion's Belt is Orion's Sword, with the Orion Nebula at its heart. This looks like a fuzzy star to the unaided eye, but binoulars or a small telescope reveal a bat-shaped cloud. A telescope with an aperture of 100 millimetres allows stars to be seen in and around the nebula, including the four stars known as the Trapezium. The brightest of these is illuminating the nebula with its ultra-violet radiation. Orion's left foot is Rigel, the constellation's brightest star, while his right shoulder is Betelgeuse, the second-brightest. To the east are the Hunting Dogs, Canis Major and Canis Minor, the larger of the two constellations containing Sirius, the brightest star in the night sky. Procyon marks the tail of the Smaller Dog. Halfway between Betelegeuse and Procyon lies the Rosette Nebula, which contains a rectangular cluster of stars. The second-brightest night-time star, Canopus, is almost overhead in the evening sky.

The Planets

Saturn rises well before dawn. It will move through Libra during 2013, and its rings will continue to become more visible as they open out to our line of sight.

Odds and Ends

Brian May's first paper since the publication of his thesis has been published on modelling infrared emission from zodiacal dust. Zodiacal dust is dust scattered along the zodiac and is what causes the zodiacal light. They have been modelling this in the infra-red emission using IRAS and COBE. They found that the majority of the dust comes from comets and asteroids with a small component of interstellar dust.

NASA deliberately crashed two of its spacecraft into the Moon on the 17th of December. Ebb and Flow, the twin probes of the GRAIL mission, had spent the previous 90 days mapping the distribution of mass in the Moon by flying in formation over its surface. NASA decided on a controlled impact at the end of their mission, and sent the two craft - each with a mass of about 200 kilograms - into a lunar mountain at 6000 kilometres per hour.